diff --git a/src/applications/diffusion/conduit/ConduitAPI_diffusion_readmequery_Method.php b/src/applications/diffusion/conduit/ConduitAPI_diffusion_readmequery_Method.php
--- a/src/applications/diffusion/conduit/ConduitAPI_diffusion_readmequery_Method.php
+++ b/src/applications/diffusion/conduit/ConduitAPI_diffusion_readmequery_Method.php
@@ -85,19 +85,30 @@
       require_celerity_resource('syntax-highlighting-css');
       $class = 'remarkup-code';
     } else {
-      // Markup extensionless files as remarkup so we get links and such.
-      $engine = PhabricatorMarkupEngine::newDiffusionMarkupEngine();
-      $engine->setConfig('viewer', $request->getUser());
-      $readme_content = $engine->markupText($readme_content);
+      // TODO: This is sketchy, but make sure we hit the markup cache.
+      $markup_object = id(new PhabricatorMarkupOneOff())
+        ->setEngineRuleset('diffusion-readme')
+        ->setContent($readme_content);
+      $markup_field = 'default';
+
+      $readme_content = id(new PhabricatorMarkupEngine())
+        ->setViewer($request->getUser())
+        ->addObject($markup_object, $markup_field)
+        ->process()
+        ->getOutput($markup_object, $markup_field);
+
+      $engine = $markup_object->newMarkupEngine($markup_field);
       $toc = PhutilRemarkupEngineRemarkupHeaderBlockRule::renderTableOfContents(
         $engine);
       if ($toc) {
-        $toc = phutil_tag_div('phabricator-remarkup-toc', array(
-          phutil_tag_div(
-            'phabricator-remarkup-toc-header',
-            pht('Table of Contents')),
-          $toc,
-        ));
+        $toc = phutil_tag_div(
+          'phabricator-remarkup-toc',
+          array(
+            phutil_tag_div(
+              'phabricator-remarkup-toc-header',
+              pht('Table of Contents')),
+            $toc,
+          ));
         $readme_content = array($toc, $readme_content);
       }
 
diff --git a/src/applications/repository/graphcache/PhabricatorRepositoryGraphCache.php b/src/applications/repository/graphcache/PhabricatorRepositoryGraphCache.php
--- a/src/applications/repository/graphcache/PhabricatorRepositoryGraphCache.php
+++ b/src/applications/repository/graphcache/PhabricatorRepositoryGraphCache.php
@@ -355,10 +355,17 @@
     // Build the actual data for the cache.
     foreach ($commit_ids as $commit_id) {
       $parent_ids = array();
-      if (isset($parents[$commit_id])) {
+      if (!empty($parents[$commit_id])) {
         foreach ($parents[$commit_id] as $row) {
           $parent_ids[] = (int)$row['parentCommitID'];
         }
+      } else {
+        // We expect all rows to have parents (commits with no parents get
+        // an explicit "0" placeholder). If we're in an older repository, the
+        // parent information might not have been populated yet. Decline to fill
+        // the cache if we don't have the parent information, since the fill
+        // will be incorrect.
+        continue;
       }
 
       if (isset($path_changes[$commit_id])) {
diff --git a/src/infrastructure/markup/PhabricatorMarkupEngine.php b/src/infrastructure/markup/PhabricatorMarkupEngine.php
--- a/src/infrastructure/markup/PhabricatorMarkupEngine.php
+++ b/src/infrastructure/markup/PhabricatorMarkupEngine.php
@@ -374,6 +374,11 @@
         $engine = self::newMarkupEngine(array());
         $engine->setConfig('preserve-linebreaks', false);
         break;
+      case 'diffusion-readme':
+        $engine = self::newMarkupEngine(array());
+        $engine->setConfig('preserve-linebreaks', false);
+        $engine->setConfig('header.generate-toc', true);
+        break;
       case 'diviner':
         $engine = self::newMarkupEngine(array());
         $engine->setConfig('preserve-linebreaks', false);
diff --git a/src/infrastructure/markup/PhabricatorMarkupOneOff.php b/src/infrastructure/markup/PhabricatorMarkupOneOff.php
--- a/src/infrastructure/markup/PhabricatorMarkupOneOff.php
+++ b/src/infrastructure/markup/PhabricatorMarkupOneOff.php
@@ -16,6 +16,16 @@
 
   private $content;
   private $preserveLinebreaks;
+  private $engineRuleset;
+
+  public function setEngineRuleset($engine_ruleset) {
+    $this->engineRuleset = $engine_ruleset;
+    return $this;
+  }
+
+  public function getEngineRuleset() {
+    return $this->engineRuleset;
+  }
 
   public function setPreserveLinebreaks($preserve_linebreaks) {
     $this->preserveLinebreaks = $preserve_linebreaks;
@@ -36,7 +46,9 @@
   }
 
   public function newMarkupEngine($field) {
-    if ($this->preserveLinebreaks) {
+    if ($this->engineRuleset) {
+      return PhabricatorMarkupEngine::getEngine($this->engineRuleset);
+    } else if ($this->preserveLinebreaks) {
       return PhabricatorMarkupEngine::getEngine();
     } else {
       return PhabricatorMarkupEngine::getEngine('nolinebreaks');